WASHINGTON (AP) — In a last-minute bid to stave off a new chapter in the East-West crisis over Ukraine, Secretary of State John Kerry warned Russia on Thursday that it faces immediate and “very serious” sanctions if it annexes Ukraine’s strategic Crimea region.

His comments echoed those of German Chancellor Angela Merkel, who hours earlier said Russia risks “massive” political and economic consequences if it refuses to soften its stance against the new government in Kiev.

The warnings from the West served as a last attempt to head off a confrontation over Crimea, which holds a vote Sunday on whether to break off from Ukraine and perhaps join Russia. The showdown has been cast as a struggle for the future of Ukraine, a country with the size and population similar to France, which is caught between its long-standing ties and traditions with Russia and more progressive and economic opportunities in the West.

Kerry was headed to London later Thursday in his last meeting with Russian Foreign Minister Sergey Lavrov before the Crimea vote. In a brief phone call Thursday morning, Kerry underscored U.S. concerns about the Crimea vote and made clear there will be costs if Russia continues to escalate the crisis, according to a senior State Department official familiar with the discussion. The official was not authorized to discuss the situation by name and spoke on condition of anonymity.

Kerry and Lavrov have spoken almost daily as the Ukraine crisis has unfolded but have yet to find any common ground.

At a Senate hearing, Kerry said Moscow should expect the U.S. and European Union to take measures against it on Monday if Russia accepts and acts on a decision by Crimea to secede from Ukraine.